The present invention relates generally to electroplating and, more particularly, to an apparatus for electroplating particles of small dimension .
The ever-increasing energy demands of the world, coupled with the threat of imminent total depletion of the earth's fossil fuel reserves, have lead to active studies of alternate sources of energy and research into developing systems and hardware for converting this energy into useful power. By way of example, solar panels have been developed which convert the radiation of the sun into heating for buildings, and the energy released by nuclear fission reactions has found application in the generation of steam and electrical power.
Controlled thermonuclear fusion reactions are yet another potential source of energy. This source is particularly attractive in that it shows promise of being almost limitless because of the naturally occurring abundance of certain fuel elements which have been successfully fused in laboratory experiments. One fusion method, which is properly called inertial confinement fusion, is a process whereby thermonuclear energy is generated when an intensely hot plasma of heavy hydrogen isotopes is inertially confined long enough for the nuclei to fuse or attain thermonuclear burn.
Initially the best fuels for inertial confinement fusion are the heavy isotopes of hydrogen called deuterium(D) and tritium(T). When deuterium tritium nuclei fuse, some of the mass of the original nuclear particles is transformed into energy. The result is a helium nucleus and an energetic (14.3 Mev) free neutron. These nuclear particles are formed with tremendous kinetic energies corresponding to the total energy released by the fusion reaction. The energetic fusion products may be harnessed to generate electrical power.
Two conditions are required to have efficient thermonuclear burn. First, thermonuclear ignition temperatures must be reached, approximately 10.sup.8 .degree.K. for deuterium and tritium. At these temperatures, the fuel matter becomes a swirling cloud of plasma and the thermal velocities of the nuclei are high enough to overcome the coulomb barrier during collision, which allows them to fuse. Secondly, the plasma must be confined long enough and at a high enough density for a significant portion of the nuclei to fuse "burn". At higher densities, the rate of burning increases and the required confinement time decreases, because the distance between nuclei is smaller and the probability of collisions is larger.
Thermonuclear ignition may be achieved by impacting a fuel pellet or target with an intense, high energy beam such as generated, for example, by a laser. Typically, laser fusion targets are minute, hollow glass micropheres having diameters on the order of 80 to 100 microns (10.sup.-6 meter), which are filled with deuterium-tritium (DT) fuel mixture.
The mechanism for accomplishing the ignition is by a spherical implosion and is based upon Newton's Third Law that for every action there is an equal and opposite reaction. The laser energy interacts with the outer surface of the microsphere causing violent ablation thereof. The ablation of the target surface produces high pressures which accelerate the remainder of the target and the fuel at the core inward toward the center of the microsphere, thereby compressing it.
Compression of the D-T fuel mixture, to as much as 10.sup.3 -10.sup.4 times the liquid density of the fuel mixture, requires relatively little energy--approximately 1% of ignition energy--provided the fuel is kept relatively cool during compression. Therefore, a significant increase in process efficiency may be realized by delaying ignition of the fuel until the rapidly moving inner region of the mixture is suddenly braked by the pressure generated in the highly compressed matter, which, in turn, heats the fuel to the ignition point.
To this end, it is advantageous to coat the surface of the glass microspheres or targets with a material having a relatively high mass and density to absorb energetic electrons and x-rays released by the laser-target interaction and thereby shield the D-T fuel mixture from premature heating. Materials known as "high-Z" materials, indicating their position on the lower portion of the periodic table, have proven satisfactory for preheat shielding. More particularly, coatings of metals such as gold, copper, nickel, platinum, and uranium appear advantageous. Moreover, in addition to acting as a preheat shield, these coatings also serve as an implosion--velocity multiplier by colliding with the glass substrate during the implosion process.
There are several methods of coating glass microspheres, including electroplating, electroless plating, chemical vapor deposition, and physical vapor deposition techniques such as sputtering. Electroplating offers many advantages over the other methods, which are more complex and more limited. For example, chemical and vapor deposition processes are inapplicable, because they heat the glass shells and drive the D-T fuel mixture out.
Furthermore, since most efficient burn occurs when the fuel is compressed into a sphere, symmetry of the target implosion is critical. Therefore, any outer coating on the target must be of uniform thickness and must be smooth and uniform to within approximately 100 nm to avoid fluid instabilities which would interfere with fuel compression.
In conventional electroplating of the tiny glass microspheres, a conductive workpiece forms a cathode which is immersed in a plating solution. An initial conductive surface called a "strike" is produced by sputtering a layer of platinum having a thickness of approximately 1-3.times.10.sup.3 Angstroms onto the surface of the nonconductive glass target spheres or by the deposition of silver thereon from an ammonical silver nitrate solution. Where the conductive microspheres with their strikes come in contact with the cathode in an electroplating cell, plating of the spheres occurs. However, unless the particles are kept in random free motion with only momentary contact with the cathode, a non-uniform coating will result, and the microspheres may even be plated to the cathode.
The problem of uniformly electroplating small parts has been addressed in U.S. Pat. No. 3,397,126 issued to F. N. Gilbert, Aug. 13, 1968. However, the system disclosed by Gilbert is unsatisfactory for the plating of initially buoyant parts, such as the glass microspheres, or those which, because of the mass of the plated material, change density with respect to the plating solution.
A process of electroplating particles with metals disclosed in U.S. Pat. No. 3,577,324 issued to J. A. Patterson, May 4, 1971 addresses the aforementioned problems by providing an apparatus wherein vibrating screens in an electroplating cell impart vibration to the plating solution to strip gaseous by-products from the particles to prevent the deposition of non-uniform surface coatings thereon. However, it has been found that in the electroplating of the glass microspheres, vibration of the solution alone or in concert with solution flow is insufficient to provide free and random motion and efficient gas scavenging of the particles. Such a system in particular fails when the difference between the density of the workpiece and the plating solution is large (as in the case of hollow glass microspheres) or when the mass of the particles is very small.
The foregoing illustrates limitations to the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative to the prior art.